The invention relates generally to a pulmonary assist device. More specifically, the invention relates to a compact pulmonary assist device used for oxygenating blood and/or removing carbon dioxide and which can be used for extended periods of time without suffering from performance degradation due to clots forming in the device.
Chronic lung diseases, such as chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF), and idiopathic pulmonary fibrosis (IPF), are one of the leading causes of death and morbidity in the world. In the United States alone, there are over 12 million chronic lung disease patients, with approximately 140,000 deaths resulting annually from the disease. The only definitive treatment for these patients is lung transplantation, yet there are only 1,700 lung transplantations per year with an estimated 50% survival at 5 years after implantation.
Those living with the disease suffer from frequent acute exacerbations of the disease state, severely reducing their quality of life. Patients ultimately experience a progressive decline in respiratory function with right heart failure from pulmonary hypertension. These patients can be maintained with oxygen therapy and pulmonary vasodilators, but not without frequent exacerbations requiring hospitalization, inflicting exorbitant cost to the patients as well as the health care system in general. However, there are no devices capable of providing long-term support for these patients and lung transplantation is the only viable treatment.
Even if a patient is designated to receive a lung transplant, many require support while waiting for donor lungs to become available. Gas exchangers, or oxygenators, used for short-term cardiopulmonary bypass are typically only used for a few hours. Other types of gas exchangers used for long-term use can include thoracic artificial lungs (TALs), devices for arterio-venous carbon dioxide removal (AVCO2R), extracorporeal membrane oxygenation (ECMO) devices, and respiratory dialysis. These devices are largely differentiated by how these devices achieve a blood supply, where the blood is returned to the patient, and whether or not a pump is used.
Mechanical ventilation is insufficient as a long-term support mechanism for patients awaiting a transplantation because it leads to muscle atrophy, lung infection, respiratory and systemic inflammation, multi-organ failure, and poor outcomes after transplantation. In the past, ECMO had been deemed insufficient as a bridge to lung transplantation due to poor outcomes during extended waiting periods. However, improvements in ECMO technology and a change in Lung Allocation Scores, which assigns priority for who will receive a transplant, drove a re-examination of this therapy. The new lung allocation system shortened wait list times, reducing the period of support required by a bridge. Median wait list times while on ECMO are thus typically 3-12 days, but many patients wait far longer.
Despite the recent advances, ECMO support is still prone to progressive complications which limit its application. For example, patients undergoing ECMO must receive regular blood product transfusions. Platelet consumption and acquired von Willebrand's disease lead to bleeding complications. Additionally, ECMO oxygenators typically fail due to clot formation within a few weeks, leading to risk of thromboembolic complications and requiring device replacement. As a result, survival to transplantation decreases with length of time on ECMO and common complications, including infections and multi-organ failure, can prohibit transplantation even if a patient survives.
Further complicating treatment, premature initiation of ECMO may lead to progressive complications and reduced survival, but if not initiated soon enough, patients may progress to irreversible organ damage. Although a few patients have been supported beyond 30 days, it is critically important to quickly find organs for patients on ECMO waiting for lung transplantation. Thus, ECMO used as a bridge to transplantation is often only considered in well-selected young patients with rapid deterioration of their lung disease in the absence of other organ dysfunction.
For example, patients with primary pulmonary hypertension were treated with a low resistance gas exchanger, the Novalung® ILA, and were successfully bridged. Although successful, the Novalung® ILA is not designed for destination therapy as it lacks sufficient 02 transfer to support patients with respiratory failure and provides only marginal unloading of the RV. Thus, when the ILA is used in the ECMO setting, patients remained on mechanical ventilation. Perhaps most importantly, the ILAs failed due to clot formation after 10-12 days on average. Thus, it is not ideally suited as either a bridge to lung transplantation or destination therapy device.
Arterio-venous carbon dioxide removal (AVCO2R) is another therapy used as a bridge to lung transplantation. In AVCO2R therapy, blood flows into a compact gas exchanger and is pumpless, which is thought to cause significantly less blood activation than ECMO and may be better suited for long-term support. Although the AVCO2R approach allows for effective CO2 transfer, it supplies less than 10% of a patient's oxygen requirements. Furthermore, the AV shunt forces the heart to increase cardiac output to maintain peripheral circulation, making AVCO2R unsuitable for patients with pulmonary hypertension and right ventricle dysfunction.
Other types of devices, such as cardiopulmonary bypass oxygenators, can assist lung function but are designed to be used for just a few hours or days. Oxygenators are used mostly to support patients undergoing open-heart surgery, for example, so the device is not intended to be used as a bridge to transplantation. Rather, the devices are designed with short-term performance in mind. While the devices perform well in these limited duration settings, the devices tend to clot after a few days. To use currently available oxygenators as a bridge to transplantation, the patient would have to swap the device every several days, which carries significant health risks.
While lung transplantation is the only viable treatment for chronic lung disease due to the drawbacks of current devices used for long-term support, many patients require only partial support to attain acceptable oxygen and carbon dioxide transfer and the unloading of their failing right ventricle. It would therefore be advantageous to develop a pulmonary assist device that provides necessary oxygen and carbon dioxide transfer levels and is capable of being used for extended periods of time.